Super-quiet drones take inspiration from silent owls

For spy tools, drones are pretty easy to spot. And hear, because
they're as loud as a gut-busting rock concert. But now the
intelligence community's research division, Iarpa, plans to start
designing a silent drone inspired by quiet, creeping, flying
owls.

Sound, after all, is the number one signature "that gives away
the location of low-altitude UAVs and gives away their presence."
Which sort of defeats the point of having a secret surveillance eye
in the sky. In some cases, you might want people to know you're
watching. At other times, you want to sneak up quietly.

But it's hard to do without sacrificing payload. The added
weight of sensors, and the ability to operate for longer periods,
comes with trading out stealthiness. Drones powered by batteries:
They're quiet, but can't stay in the air for long. Then there's the
added noise caused by airflow generated from propellers, and
noise from gasoline or diesel engines (not counting batteries),
with their moving pistons, turbofan and gears.

Iarpa wants to keep these efficient and relatively noisy engines for normal flight. But when the drone
needs to be stealthy, its operator would switch to battery
power, like a hybrid car. That means -- for the duration of battery
flight -- the noisy gears would shut off. The propellers would also
likely be ducted, which would mean less noise from vortices whipped
up by the propellers and fewer moving parts. Likely, the drone will
take off vertically.

The agency doesn't expect the drone to stay ultra-quiet for more
than 30 minutes, though, at which point the gasoline-powered
turbine engine would switch back on, recharging the batteries. Not
enough for (say) a sustained surveillance operation, but quiet
enough to take a peek at an enemy without being noticed.

The first step is keeping the sound levels in battery mode below
100 decibels, about equivalent to a chainsaw when up close. But
give perhaps a few thousand feet of distance, and the noise
drops. Iarpa also wants to reduce noise by cutting down the
drone's "phon curve" -- or the level of sound pressure interpreted
by the human ear. Iarpa plans to start by testing the sound levels
of an uninstalled version of the engine.

Iarpa's owl drone is also not the first talk of an owl-inspired
aircraft. NASA has looked into the owl's stealthy feathers for inspiration. But it may take years before
owl-based aircraft migrate into service.

Comments

Nice to see this jubilant PR campaign for drones will always be given a free ride in Wired. Hell, it's not as if these things are used for anything morally dubious, say, extra-judicial assassination of so-called 'militants, in contravention of international law. Not to mention the deaths of civilians, the cowardice involved in PlayStation-style execution from thousands of miles away, etc. etc.